With rapid propagation of various types of electronic devices, there is a tendency that these devices are increasingly produced in compact and lightweight design. In particular, this tendency toward compact and lightweight design is more remarkable in the electronic devices of portable type to be used in camera-integrated video tape recorder, portable telephone set, note-book personal computer, palm-top computer, etc.
In such tendency toward compact and lightweight electronic products, the component parts for these devices are also increasingly produced in compact and lightweight design. The means to mount the electronic components are also changing from the technique for inserting and soldering pins, i.e. electronic parts, to be used in through-hole on conventional type printed board to surface mounting technology (SMT), i.e. the technique for mounting and soldering electronic parts on land of electro-conductive pattern on printed board.
The electronic parts used in this SMT are generally called surface mounting devices (SMD). Not only semiconductor components, but also capacitor, resistor, inductor, filter, etc. are included in them. Among them, small components such as capacitor and resistor are called chip components. The most representative of them is multi-layer ceramic capacitor.
FIG. 1 shows a partially cutaway perspective view showing structure of a multilayer ceramic capacitor.
The multilayer ceramic capacitor 1 is in form of rectangular parallelopiped, and terminal electrodes 3 and 4 are mounted on a pair of opposed surfaces of multilayer ceramic capacitor body 1, which is in form of rectangular parallelopiped.
In the multi layer ceramic capacitor main unit 2, four internal electrodes 5, 6, 7 and 8 are formed among laminated BaTiO.sub.3 dielectric layers 9, 10 and 11, and dielectric material layers 12 and 13 made of dielectric material are laminated on upper surface of the internal electrode and lower surface of the internal electrode 8.
Every other of the internal electrodes 5, 6, 7 and 8 are connected to terminal electrodes. That is, the internal electrodes 6 and 8 are connected to a terminal electrode 4, and the internal electrodes 5 and 7 are connected to another terminal electrode 3. As a result, a capacitor is formed, which is connected in parallel between the internal electrodes 5 and 6, between the internal electrodes 6 and 7, and between the internal electrodes 7 and 8.
Each of the terminal electrodes 3 and 4 comprises a conductive layer, on which conductive paste containing glass frit is coated or printed and baked, and a plated layer coated on it or a metal cap press-fitted on it.
To manufacture multilayer ceramic capacitor, electrode paste to serve as an internal electrode is printed on a dielectric ceramic composition sheet, a plurality of such sheets are laminated and thermally pressed, the laminated product thus prepared is sintered in the air and a terminal electrode is mounted on it.
In this manufacturing method, the electrode paste to serve as the internal electrode of the capacitor and the dielectric ceramic composition are fired at the same time. For this reason, the material to be used as the internal electrode must have such property that the electrode is formed at the sintering temperature of the dielectric ceramic composition and there occurs no oxidation or no reaction with the dielectric ceramic composition when heated in the air.
As the material to meet the above requirements, noble metal such as platinum, palladium, etc. have been used in the past. However, these noble metals are very expensive and constitute major cause for the high cost of the multilayer ceramic capacitors.
In this connection, attempts have been made to use base metal such as nickel as internal electrode, while nickel is oxidized when it is fired in oxidizing atmosphere and also reacts with the dielectric ceramic composition. This hinders the formation of electrode.
A method to fire nickel in non-oxidative atmosphere to prevent oxidation is disclosed in the specification of U, S. Pat. No. 4,241,378, while, in this method, dielectric ceramic composition is reduced and specific resistance is extremely lowered. Thus, it is not suitable for the practical use as capacitor.
As a dielectric ceramic composition having satisfactory dielectric property such as dielectric constant (relative permitivity), BaTiO.sub.3 dielectric ceramic composition containing CaZrO.sub.2, MnO, etc. is disclosed in Japanese Patent Laid-Open Publication 62-2408, whereas this dielectric ceramic composition is fired in non-oxidizing atmosphere when it is reduced and has shorter life time.
The multilayer ceramic capacitor is manufactured through the following processes.
(1) Raw materials are weighed and blended so that composition after firing complies with the predetermined blending ratio.
(2) Wet mixing and pulverizing are performed.
(3) Dehydration and drying are performed.
(4) Adequate quantity of organic binder is added, and it is mixed and turned to enameled.
(5) This is coated on film by doctor blade method to form dielectric ceramic composition sheet.
(6) On the dielectric ceramic composition sheet thus prepared, nickel paste to serve as a material for internal electrode is formed by printing.
(7) The product is laminated and thermally pressed to obtain a multilayer product.
(8) The product is cut into a predetermined shape.
(9)Binder removal processing is performed.
(10) While controlling oxygen partial pressure, the product is fired.
(11) Re-oxidation is performed in neutral atmosphere.
(12) Terminal electrode is mounted.
As a non-reducing dielectric ceramic composition having longer life time, a dielectric ceramic composition containing {Ba.sub.A, Ca.sub.(1-A) }SiO.sub.3 (where 0.ltoreq.A.ltoreq.1) (hereinafter referred as "BCG") as an additive to BaTiO.sub.3, MnO, and Y.sub.2 O.sub.3 is disclosed in Japanese Patent Application 3-18261. However, in the multilayer ceramic capacitor produced using this dielectric ceramic composition, capacitance is varied due to temperature change.
In addition to the above, non-reducing dielectric ceramic composition is described in Japanese Patent Laid-Open Publications 61-248304 and 57-71866, U.S. Pat. No. 4,115,493, "Dielectric Materials for Base-Metal Multilayer Ceramic Capacitors" (Proceedings of the Electronics Division Fall Meeting, the American Ceramic Society, Oct. 13-16, 1985) by Y. SAKABE, T. TAKAGI, and K. WAKINO.